1. Field of the Invention
This invention relates to a system and method for exposing holograms, and more particularly, relates to a system and method for forming master holograms with a relatively large translational motion of a point source of illumination which is gradual and follows the angular scanning of the point source.
2. Description of Related Art
High quality holographic optical elements are used in diffraction optics display systems, such as a Head Up Display (HUD) for advanced aircraft, helmet mounted displays, laser eye protection devices, narrow band reflective filters, and holographic high gain screens for simulators. These are only a few of the many uses of high quality holograms. Problems can arise in providing economical mass production of reflective holographic optical elements where the production units are "copies" of either a master reference object or a master hologram, which may, for example, provide an aspheric reflective wavefront.
Diffraction optic display systems utilizing a hologram can experience a degradation of the holographic images as a result of the effects of spurious reflection and transmission hologram recordings that may be generated during the holographic replication process. The spurious noise holograms have been found to be generated by reflections from surfaces which are interfaces of materials having a different refractive index such as air/glass, or imperfect indexed matched interfaces of the surfaces of the recording cover plate, the holographic substrate, and the recording medium. These reflections can combine with the primary holographic beams at the recording medium to form both spurious reflection hologram recordings and spurious transmission hologram recordings. As a result, a subsequent display system can create ghost images from the spurious reflection hologram recordings and rainbow-like flare patterns from the spurious transmission hologram recordings.
The prior art has attempted to address these problems in numerous different ways. One approach has been to minimize the difference in indexes of refraction by attempting to match the indexes of refraction with an index matching fluid, such as mineral oil. Attempts have been made to immerse a recording module in an index of refraction matching oil bath. Another approach has been to form a hologram with energy beams impinging the recording film supporting elements at the Brewster's angle (the angle for which light of a given polarization has a very low reflectivity).
U.S. Pat. Nos. 4,458,977, 4,458,978, and 4,456,328 disclose prior art approaches to eliminate the noise caused by a glass/air interface of an outer surface cover plate by moving the cover plate to change the phase of the reflected rays relative to the primary beams during the recording period so that spurious interference patterns are not formed. The rate of movement or phase change in accordance with these solutions is a function of the exposure time which itself is a function of the sensitivity of the recording medium. The total amplitude of the movements is made sufficient to produce a phase change of one or more half wavelengths in the reflected noise beams to nullify such spurious recorded interference patterns. These approaches have been proposed to solve the problems involved in the manufacturing of reflective holographic optical elements for use in head up displays.
Generally, prior art solutions employ a layer of index matching fluid, such as an appropriate mineral oil, which can vary in thickness during the cover movement. The required relatively thick image degrading layer of index matching fluid has the capacity to degrade the image of the reference object, such as an aspheric mirror, create moving striations causing fringe degradation, and furthermore requires frequent cleaning or replacement of the oil. In a double beam recording system oil instability requires days of stabilization before a successful holographic exposure can be made. A master aspheric mirror single beam exposure system still requires many hours of stabilization and the use of relatively skilled labor.
Additionally, in the prior art approaches, generally only the outer surface, that is, the glass/air interface elements, could be provided with an antireflective coating. If an inner surface was required to be coated for optimum use in air, the antireflective coating had to be added at a later time after the exposure, such as by adding an antireflective coated cover glass, which would add further weight, or by depositing a standard anti-reflective coating, which would frequently thermally destroy the hologram, or by depositing a cold anti-reflective coating, which would be less efficient and more fragile. Finally, this example of prior art required a piezoelectrically controlled exposure cover that had to be appropriately mounted and calibrated prior to exposure.
U.S. Pat. No. 4,478,490 discloses an alternative method of reducing coherent noise content through the modulation of the position of an apodizer in the optical path during an exposure. The motion of the apodizer creates a condition permitting the amplitude of the wavefront to be modified to alter a point source response, that is, to change the point spread function, whereby the fringe patterns created by the apodizer are unstable and hence reduce the noise content of the transmitted radiation.
Another prior art attempt to remove noise has been the use of a laser source without an etalon to reduce noise holograms from a surface further away than the coherence length of the laser (approximately 2 inches for a large argon laser). While this method can reduce noise, it is applicable only in cases in which the exposure apparatus surfaces are closely spaced, such as approximately one-quarter inch, such as in a HUD-type hologram with an aspheric mirror surface.
The prior art has frequently recognized the desirability of reproducing copies from a master hologram. A theory of such copying of holograms is set forth in Brumm, "Copying Holograms," Applied Optics, Volume 5, No. 12, page 1946, December 1966. Reference is also made to U.S. Pat. No. 3,758,186, U.S. Pat. No. 3,639,031, U.S. Pat. No. 3,647,289, U.S. Pat. No. 4,312,559, and U.S. Pat. No. 4,530,564 to disclose other methods of copying holograms.
In modern aircraft, there is frequently minimal space in the cockpit. This limitation requires that any optical system be folded and compressed for a head up display. As a result, complex aspheric reflecting mirrors are needed and holographic aspheric mirrors represent a lightweight and efficient solution to this requirement. As mentioned above, conventional glass or metal aspheric mirrors have been fabricated for use as a master reference in producing HUD holograms. This approach involves a lengthy and expensive procedure of grinding an aspheric mirror to the subjective requirements of a particular head up display. Furthermore, the nonspherical surface of such a master mirror limits how close the aspheric reference member can be placed relative to the recording material (unless it is also on an aspherical surface) for replicating the aspheric diffraction grating in the recording medium. .
Another method to generate the master mirror that is known in the prior art is to provide computer generated holograms. In this matter, the design of the desired wavefront is mathematically described and a computer then forms a two-dimensional amplitude hologram representative of that wavefront. The computer can drive a printer to produce the desired diffraction grating on a substrate or alternatively create the grating by electronic or chemical procedures. A problem in using a computer generated hologram as an initial imaging source for the fabrication of a holographic HUD combiner is the noise in the computer generated master hologram. This noise is present in the form of a general nonuniformity of brightness and in multiple order scattering. The computer generated hologram has multiple order noise because the fringe pattern is generally formed as abrupt discrete units rather than sinusoidally varying as in an ideal holographic recording of a laser interference pattern.
It would be desirable to provide a method and apparatus for the reproduction of multiple hologram optical elements in an economical and efficient manner, to include scanning the exposure with a single mode polarization preserving fiber optic to provide the primary beam, with a lateral dithering motion of the fiber optic to reduce noise, with a relatively small distance between the photosensitive material forming the reflective holographic optical element and the reflective element which provides the second beam, and a relatively large translational motion which is gradual and generally follows the angular scanning motion.